In 2003, Cassidy made her television and acting debut appearing in an episode of the Lifetime Television Drama television series The Division. Cassidy has continued to guest appear in television shows such as Celebrity Apprentice 4, Listen Up! and New Girl. In 2005, Cassidy appeared in a recurring role on The WB television series 7th Heaven over the course of four episodes. That same year, she also appeared in the UPN Teen Drama television series Sex, Love & Secrets.
In 2007, Cassidy appeared on The CW supernatural drama television series Supernatural. Cassidy played the role of Ruby, a demon killer. Making her debut in the show's third season, Cassidy appeared in over six episodes. In 2008, though her character returned for the show's fourth season, Cassidy was replaced by actress Genevieve Cortese.
In October 2008, Variety announced Cassidy was cast in the CBS horror mystery mini-series television series Harper's Island. Cassidy portrayed the role of Patricia "Trish" Wellington, a bride-to-be. The series centered on a group of family and friends who gather on an island for a wedding on an island known for its sinister past full of gruesome murders. In each episode, at least one character is killed off.
Cassidy and her fellow cast members were not told about their characters' deaths until the day they received a script. The series premiered on April 9, 2009 to 10.21 million viewers. Despite ratings failing to maintain the pilot's success, CBS aired the remainder of the series with the series finale airing on July 11, 2009.
That same year, The CW cast Cassidy in the Drama television series Melrose Place. The series is a reboot of the 1990s series of the same name.Cassidy portrays the role of Ella Sims a publicist who resides in a West Hollywood apartment complex and centers on its residents.The series premiered on September 8, 2009 to 2.31 million viewers.
Despite mixed reviews, Cassidy's performance was praised by critics and was cited as "the best thing about the Melrose reboot" by New York and has been cited as "the best thing about the Melrose reboot" by magazine. The CW cancelled the series in May 2010 due to low ratings.